@lyndsey33,
I'm not a legal expert at all, but if the facts are as you state them, it seems to me that you would be within your rights to say that you've changed your mind and do in fact want to parent your child, and that at this point, you'd have a good chance of getting her back.
If you're native American and the potential adoptive parents are native American, I'm assuming you're in the US. And I know that it varies from state to state, but I seem to remember that in most states, most birth mothers have at least thirty days after the birth of the child to decide if their decision to place for adoption is indeed what they really want to do or not- during which time that baby is not totally legally free for adoption. This makes perfect sense to me -how can someone know before actually having the baby and handing him/her over that they will be able to live with this decision?
But I do have one question- if you never signed any papers or consent, how was the 'agreement' manifested with the couple who want to adopt your baby?
I hope you get your wish and that everyone involved - including the couple who wish to adopt your child - understand what a hard and wrenching decision you had to make and that you are entitled to change your mind.
I know I wouldn't want to take a baby from a mother who had decided she does want to parent her own child. Maybe the potential adoptive parents will feel the same way - if they know that you do want your baby - they'll understand it's your right first and foremost to parent that child. She is YOUR baby.
But I would let the appropriate people know as soon as possible. I wouldn't let another minute go by without starting the process to get your baby back with you. Time is on your side right now, but with every minute that passes - so does your legal right to have your daughter back with you.